A typical example of a traveling device capable of suction-adhering to a travel surface having curved faces, such as the inside or outside surface of a pipe or the surface of a ship hull, by the pressure of an ambient fluid such as air or water, and of moving along the travel surface, while performing work such as sandblasting, is the device disclosed in the specification and drawings of U.S. Pat. No. 4,095,378. The device has a pressure reduction housing mounted on a frame, a moving means mounted on the frame, a suction-adhering seal mounted on the pressure reduction housing and defining a pressure reduction space in cooperation with the pressure reduction housing and the travel surface, and a pressure reducing means for discharging a fluid from the pressure reduction space to decrease the pressure inside the pressure reduction space. The moving means is composed of a plurality of wheels (e.g. 4 wheels), or a plurality of endless tracks including a plurality of belt pulleys (e.g. 2 endless tracks each with 2 belt pulleys). The traveling device is equipped with a working unit such as sandblasting equipment which directs a jet of a polishing and cleaning material at the transfer surface.
In the above-described conventional traveling device, when the pressure reducing means is actuated, fluid inside the pressure reduction space is discharged to the outside. Owing to the difference in fluid pressure between the inside and outside of the pressure reduction space, the fluid pressure acts on the pressure reduction housing, and is then transmitted to the travel surface via the wheels or belt pulleys. Under this fluid pressure, the device is caused to suction-adhere to the travel surface. In this state of suction-adhesion, when the wheels or belt pulleys are driven by a driving means such as an electric motor, the device moves along the travel surface by the action of the wheels or endless tracks.
In the foregoing traveling device which suction-adheres to a travel surface having curved faces, such as the inside or outside surface of a pipe or the surface of a ship hull, by the pressure of an ambient fluid, and moves along the travel surface, while performing work such as sandblasting, the frame on which the moving means is mounted has a rigid structure whose shape substantially does not change flexibly. Therefore, in the traveling device equipped with, say, 4 wheels as the moving means, i.e., the traveling device having 2 wheels on each side of the frame, one of the four wheels separates from the travel surface. More specifically, if the device sits on the outside surface of, say, a cylindrical pipe, and if the central axis in the traveling direction of the device is on a plane obliquely intersecting the central axis of the pipe, one of the four wheels departs from the travel surface. Owing to this departure, the posture of the device is always unstable. Not all of the four wheels being in contact with the travel surface results in an insufficient driving force. Such technical problems exist also with a conventional traveling device which moves while performing work such as sandblasting, but which lacks means for suction-adhering to the travel surface by the pressure of an ambient fluid. Furthermore, in the aforementioned traveling device which suction-adheres to a travel surface by the pressure of an ambient fluid, and moves along it, the sealing function of the suction-adhering seal may be hampered, owing to the separation of one of the four wheels from the travel surface. One could easily understand that these problems occur for the same reasons as stated earlier, in connection with the aforementioned traveling device equipped with two endless tracks including a total of 4 belt pulleys as moving means.
In the case of the aforesaid traveling device which suction-adheres to a travel surface by the pressure of an ambient fluid, and moves along it, and which has 4 wheels as the moving means, namely, a traveling device of the type having 2 wheels on each side of the frame, the following phenomenon may arise: During the upward movement of the device on a vertical travel surface such as the wall surface of a ship hull, if a projection extending in a horizontal direction, such as a different-level weld line formed by a laminate of welded steel plates, exists on the travel surface, the device has no sufficient driving force to pass over the projection, with the result that the wheels may rotate at the projection, without imparting a forward movement. Especially when a coating with a small coefficient of friction is present on the travel surface, making the wheels easily slip there, the ascent of the device is impeded at the projection, causing the wheels to run idly. The reason is as follows: In the aforesaid traveling device, the rotating shafts of the four wheels are arranged on the same plane which is at right angles to the traveling direction of the device and which perpendicularly intersects the travel surface. Hence, as the device moves upwards on the vertical travel surface in the vertical direction, 2 of the wheels of the device simultaneously encounter the projection. Thus, the driving force necessary for the device to pass across the projection is twice the driving force required when each one of the four wheels passes over the projection. It will be easily understandable that this problem occurs for the same reason, in regard to the aforementioned traveling device provided with two endless tracks including, say, four belt pulleys, as the moving means. Such a technical problem is also posed with a conventional traveling device which moves while performing work such as sandblasting, but which lacks means for suction-adhering to the travel surface by the pressure of an ambient fluid.
The conventional traveling device, which suction-adheres to the travel surface by the pressure of an ambient fluid and moves along it, may face the following situation: When the device moves while suction-adhering to the travel surface having curved faces, such as the inside and outside surfaces of pipes or those of a ship hull, it may encounter a travel surface having a great curvature in excess of a critical curvature within which travel with suction-adhesion remains possible. In this case, the suction-adhering seal of the device reaches its limit of flexible deformation according to the curvature of the travel surface. This may destroy the sealing function of the suction-adhering seal. Thus, the degree of vacuum of the pressure reduction region of the device may rapidly fall, causing the device to separate from the travel surface abruptly.